Hermit crabs are anomuran decapod crustaceans of the superfamily Paguroidea that have adapted to occupy empty scavenged gastropod shells to protect their fragile abdomens. There are over 800 species of hermit crab, most of which possess an asymmetric abdomen concealed by a snug-fitting shell. Hermit crabs' soft (non-calcified) abdominal exoskeleton means they must occupy shelter produced by other organisms or risk being defenseless.

The strong association between hermit crabs and their shelters has significantly influenced their biology. Almost 800 species carry mobile shelters (most often calcified snail shells); this protective mobility contributes to the diversity and multitude of these crustaceans, which are found in almost all marine environments. In most species, development involves metamorphosis from symmetric, free-swimming larvae to morphologically asymmetric, benthic-dwelling, shell-seeking crabs. Such physiological and behavioral extremes facilitate a transition to a sheltered lifestyle, revealing the extensive evolutionary lengths that led to the superfamily's success.

The hermit crabs of Paguroidea are more closely related to squat lobsters and porcelain crabs than they are to true crabs (Brachyura). Together with the squat lobsters and porcelain crabs, they all belong to the infraorder Anomura, the sister taxon to Brachyura.

However, the relationship of king crabs to the rest of Paguroidea has been a highly contentious topic. Many studies based on their physical characteristics, genetic information, and combined data demonstrate the longstanding hypothesis that the king crabs in the family Lithodidae are derived hermit crabs descended from pagurids and should be classified as a family within Paguroidea. The molecular data has disproven an alternate view based on morphological arguments that the Lithodidae (king crabs) nest with the Hapalogastridae in a separate superfamily, Lithodoidea. As such, in 2023, the family Lithodidae was placed back into Paguroidea after having been moved out of it in 2007.

Nine families are formally recognized in the superfamily Paguroidea, containing around 1200 species in total in 148 genera.

The placement of Paguroidea within Anomura can be shown in the cladogram below, which also shows the king crabs of Lithodidae as sister taxon to the hermit crabs of Paguridae:

The fossil record of in situ hermit crabs using gastropod shells stretches back to the Late Cretaceous. Before that time, at least some hermit crabs used ammonite shells instead, as shown by a specimen of Palaeopagurus vandenengeli from the Speeton Clay Formation, Yorkshire, UK, from the Lower Cretaceous, as well as a specimen of a diogenid hermit crab from the Upper Jurassic of Russia. The earliest record of the superfamily extends back to the earliest part of the Jurassic, with the oldest known species being Schobertella hoelderi from the late Hettangian of Germany.

Hermit crabs can be informally divided into two groups: aquatic hermit crabs and terrestrial hermit crabs.